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Monitored water quality

The data we used for the state and trend analyses in this section come from regional councils (data from monitored rivers, lakes, and groundwater sites), NIWA’s National River Water Quality Network, and GNS Science’s National Groundwater Monitoring Programme.

Percentages may not add up to 100 percent, as they are rounded to the nearest percent.

Regional compared with national level reporting

Two issues affect national-level reporting of water quality, which is based on a collection of data from regional councils.

  1. Regional council monitoring networks in New Zealand are over-represented by sites in the pastoral class, and under-represented by sites in the native class (Larned & Unwin, 2012). This is because most councils monitor areas with known or suspected issues for management purposes. For example, rivers in low-lying and hilly areas in the North and South islands are well represented, while mountainous areas in the South Island and parts of the central North Island are not. This means our current monitoring networks do not provide a representative picture of water quality for all New Zealand rivers.
  2. Although our data are sourced from regional councils, we adjusted some datasets to ensure our reports are nationally consistent. The adjustments may include omitting information produced by non-comparable methods. As a result, our evaluations may differ from those based on the original data. If you want detailed regional-level information, we recommend you review the relevant regional council’s environmental reports.

We have similar issues in our lake and groundwater quality data.

Very few lakes are monitored in a consistent way. In this report, we report on between 48 and 76 lake sites, depending on the water quality variable.

Inconsistencies in the monitoring of groundwater quality include different sampling methods and frequencies of sampling that make it difficult to create a national picture of the state of, and trends in, groundwater quality. The National Groundwater Monitoring Programme network consists of 106 sites monitored quarterly by GNS Science using a consistent set of variables. The number of sites sampled, sampling frequency (from monthly to annually), sampling methods, and the variables measured for groundwater quality vary between regions.

Due to these issues, we do not show spatial results (ie on maps) of water quality from monitoring sites. Instead, we use numbers and proportions of monitoring sites to report on water quality at a national scale and rely on the water quality models in the previous section to show spatial patterns. If you are interested in seeing spatial representations of water quality from monitoring sites, see our dynamic maps at Environmental indicators Te taiao Aotearoa – Fresh water [Stats NZ].

Classification of water quality sites

We classified the results from the monitoring of river water quality by land-cover class under the River Environment Classification (Snelder & Biggs, 2002). We used four land-cover classes: pastoral, urban, exotic forest, and native. The River Environment Classification assigns land-cover classes based on the land cover in the upstream catchment that is presumed to dominate conditions in surface water. Monitoring sites assigned to a particular land-cover class may have other land-cover classes upstream, and this variation in land cover can contribute to variation in water quality between sites. The majority of the monitoring sites we report on for river water quality are in the pastoral land-cover class. Because of this, the results we present in the sections below are for sites in the pastoral class, with supporting graphs that show results for all four land-cover classes.

We did not classify lake water quality sites in this report; we considered classifying them by lake type but the small sample size in each class made the accuracy of this inter-class comparison questionable.

We did not classify our groundwater quality sites in this report.

In future reports, we will explore options for classifying our river, lake, and groundwater sites.

Trend assessments

We used two trend-assessment methods for water quality: one for rivers and lakes, and another for groundwater. We used different trend tests because we trialled a new approach for river and lake water quality (Larned et al, 2015).

1. Trends for river and lake water quality variables were inferred with 95 percent confidence using the Relative Seasonal Sen Slope Estimator (Larned et al, 2015). A trend can either be ‘upwards’, ‘downwards’, or ‘insufficient data to determine trend direction’ (which we refer to as ‘indeterminate’). Data used to determine the trends for river water quality (except for macroinvertebrate community index) were adjusted for the influence of variation in river flow, because values may be correlated with flow. Where flow measurements were not available, flow was estimated using theTopNet national hydrological model (Larned et al, 2015).

2. Stats NZ assessed the trends for groundwater quality using the non-parametric Mann-Kendall test for the presence of positive or negative trends. A p-value of <0.05 was used to determine whether the trend was statistically significant (if not significant, we refer to the trend as indeterminate).

An indeterminate trend does not mean conditions have not varied over the assessment period.